The present invention relates to niobium-based anodes for electrolytic capacitors and also to a process for producing such anodes.
In the literature, in particular, the acidic earth metals niobium and tantalum are described as starting materials for the production of such anodes and capacitors. The anodes are produced by sintering finely divided metal powder to produce a structure having a large surface area, oxidation of the surface of the sintered body to produce a nonconducting insulating layer and application of the counter electrode in the form of a layer of manganese dioxide or of a conductive polymer.
Hitherto, only tantalum powder has achieved industrial significance for capacitor production.
The essential specific properties of such capacitors are determined by the specific surface area, the thickness of the oxide layer d forming the insulator and the relative permittivity xcex5r. The capacitance C is consequently calculated as follows:                                           C            =                                          ϵ                0                            ⁢                                                ϵ                  r                                ·                                  A                  d                                                              ⁢                      
                    ⁢          where                ⁢                  xe2x80x83                                    (        I        )                                          ϵ          0                =                              0.885            ·                          10                              -                11                                              ⁢                      F            /            m                                              (        II        )            
denotes the dielectric field constant and A denotes the capacitor surface.
The insulating oxide layer of the capacitor is conventionally produced electrolytically by immersing the sintered niobium or tantalum structure that forms the capacitor anode in an electrolyte, conventionally dilute phosphoric acid, and applying an electrical field. The thickness of the oxide layer is directly proportional to the electrolysis voltage, which is applied with initial current limitation until the electrolysis current has fallen to 0. Conventionally, the oxide layer is produced at an electrolysis voltage (xe2x80x9cforming voltagexe2x80x9d) that is equal to 1.5 times to 4 times the intended operating voltage of the capacitor.
The relative permittivity of tantalum pentoxide is conventionally specified as 27 and that of niobium pentoxide is conventionally specified as 41. The growth in thickness of the oxide layer during forming is about 2 nm/V forming voltage for tantalum and about 3.7 nm/V for niobium, with the result that the higher relative dielectric constant of niobium is compensated for by the greater thickness of the oxide layer for an identical forming voltage.
The capacitors are miniaturized by increasing the specific surface area by using finer powders for producing the sintered structure and reducing the sintering temperature.
The required thickness of the insulating oxide layer places limits on the miniaturization of the capacitors, i.e. on the increase in the specific capacitance, since a sufficiently conductive phase for current conduction and limitation of the resultant ohmic heat must still be present within the oxidized sintered structure. The oxidation tendency consequently increases with increasing miniaturization of the capacitors. This applies, in particular, to niobium capacitors, which, compared with tantalum capacitors, require a thicker insulating oxide layer for an identical forming voltage.
It has now been found that the capacitor properties can be advantageously modified if, during the forming, an electrolyte is used that contains a multidentate organic acid anion that forms stable complexes with niobium. Suitable organic acids for use in the forming electrolyte are, for example, oxalic acid, lactic acid, citric acid, tartaric acid, phthalic acid, the preferred acid anion being the anion of oxalic acid.
The invention relates to an anode having a niobium-based barrier layer comprising (a) a niobium metal core, (b) a conducting niobium suboxide layer, and (c) a dielectric barrier layer comprising niobium pentoxide. The invention also relates to a process for producing an anode for a capacitor comprising sintering niobium metal powders and electrolytically producing a dielectric barrier layer on a surface of a sintered body, in which the barrier layer is produced with an electrolyte that contains an aqueous solution of an organic acid containing an anion. These and other features, aspects, and advantages of the present invention will become better understood with reference to the following description and appended claims.
The invention relates to an anode having a niobium-based barrier layer comprising (a) a niobium metal core, (b) a conducting niobium suboxide layer, and (c) a dielectric barrier layer comprising niobium pentoxide. The invention also relates to a process for producing an anode for a capacitor comprising sintering niobium metal powders and electrolytically producing a dielectric barrier layer on a surface of a sintered body, in which the barrier layer is produced with an electrolyte that contains an aqueous solution of an organic acid containing an anion.
The electrolyte may contain the organic acid in aqueous solution. Preferably, a water-soluble salt of the organic acid is used. Suitable as cations are those that do not adversely affect the oxide layer and whose complex formation constant with the corresponding acid anion is lower than that of niobium with said acid anion, with the result that niobium ions can be replaced by the corresponding metal ions. Preferred are cations that beneficially affect the capacitor properties when they are incorporated in the oxide layer. A particularly preferred cation is tantalum.
Preferred as forming electrolyte, in particular, is an aqueous solution of tantalum oxalate. The invention is described below using the example of tantalum oxalate without restriction of the generality.
The forming process according to the invention achieves capacitors with a capacitance increased by up to about 50% compared with conventional forming in dilute phosphoric acid. The specific leakage current is below 0.5 nA/xcexcFV.
It was found that the capacitance-increasing effect is the greater, the higher the conductivity of the electrolyte during forming.
The electrolyte concentration is preferably adjusted in such a way that the conductivity of the electrolyte is from about 1.5 to about 25 mS/cm, particularly preferably from about 5 to about 20 mS/cm and, in particular, preferably from about 8 to about 18 mS/cm. 6. In one embodiment, the electrolyte has a conductivity ranging from about 0.15 to about 25 mS/cm.
During forming, it is advantageous to limit the forming current initially to 30 to 150 mA/m2 of anode area. In this connection, forming currents limited to lower values are preferably used in the case of electrolytes with lower conductivity. In the case of higher electrolyte conductivity, the forming currents can be set in the upper range.
The capacitance-increasing effect according to the invention is attributed to a specific surface removal of niobium from the anode structure during forming. Niobium contents in the region of a few wt % of the anode structure used are found in the forming electrolyte after forming. Typically, the niobium dissolution during forming is 3 to 5 wt % and in some cases even up to 10 wt % of the anode structure. Obviously, the surface removal takes place specifically in such a way that the effective capacitor area is increased compared with forming in dilute phosphoric acid. During conventional forming in phosphoric acid, pores are sealed or blocked as a result of the increase in volume due to the formation of the oxide layer, with the result that the effective capacitor surface area is reduced. Obviously, the organic acid anion attacks precisely those surface regions that limit particularly narrow pore channels.
A further advantageous effect of the invention is that the oxide layer is formed in two layers: an outer pentoxide layer that forms the insulating layer and an inner, conductive suboxide layer situated between pentoxide layer and metal core. SEM micrographs of fracture facets of fractured formed anodes reveal very thick oxide layers that correspond to a layer thickness growth of 5 nm/V forming voltage or more, in some cases only a vanishingly small metal core being enclosed. Under the light microscope, color differences (violet/green) reveal that the oxide layer is composed of two adjacent sublayers. The suboxide layer acts as a barrier for oxygen diffusion out of the pentoxide layer and consequently contributes to the long-term stability of the anode.
A further advantage of the invention is that the cation of the electrolyte solution is deposited to a small extent on the anode surface and, because of the diffusion kinetics, is incorporated in a stabilizing manner in the oxide layer in competition with the diffusion of oxygen into the anode and of niobium to the anode surface during oxidation. Thus, tantalum, which does not form stable suboxides, is suitable for stabilizing the pentoxide layer. Since niobium has the higher site interchange probability compared with tantalum (see, for example, J. Perriere, J. Siejka, J. Electrochem. Soc. 1983, 130(6), 1260-1273) the niobium is capable of xe2x80x9cjumping overxe2x80x9d surface-deposited tantalum during the oxidation, with the result that tantalum does not apparently migrate inwards within the growing oxide layer. It accumulates at the inside of the pentoxide layer and stabilizes it. In the anodes formed according to the invention, tantalum contents are found of about 1500 to about 10000 ppm, predominantly of about 3000 to about 6000 ppm, relative to the anode, the tantalum being concentrated in the pentoxide layer. Some of the capacitance-increasing effect of the present invention is probably attributable to a beneficial effect on the pentoxide layer thickness growth and, optionally, the permittivity.
The invention also relates to anodes having a barrier layer for niobium-based capacitors, comprising a metallic niobium core, a conducting niobium suboxide layer and a dielectric barrier layer of niobium pentoxide. Preferably, the niobium suboxide layer has a thickness of at least about 30 nm, particularly preferably at least about 50 nm.
Particularly preferred anodes according to the invention have a pentoxide barrier layer containing from about 1500 to about 5000 ppm of tantalum, relative to the anode. In one embodiment, the anode according to the invention has a tantalum content in the dielectric barrier layer ranging from about 1500 to about 12,000 ppm, relative to the anode.
The invention is further described in the following illustrative examples in which all parts and percentages are by weight unless otherwise indicated.